If you bought it intending to hold it and something unforeseen is causing you to change your intent then that is a different story and a 1031 might be appropriate.  

The other factor is, stocks COULD drop significantly due to an unforeseen circumstance such as terrorism.

How often to do come across an unforeseen repair like a foundation issue etc?

I know that I could continue investing in my 401K, work through to retirement and finish strong (barring any unforeseen economic catastrophe), but the sexiness of real estate brings more investment diversification, through:1. 

I love what I do, but I know it won't last forever and there is always the risk of an unforeseen injury or underperformance that can hinder my earning ability.

Take just enough for all of your obligations and make sure you have enough cash flow left to cover any unforeseen expenses.

Even if I invest €20k over 10 years to offset unforeseen expenses (vacancies or misc repairs), I'll still receive nearly a 30% CoC ROI.

You'll have to throw money into the kitchens and bathrooms (not cheap) and leave room for holding costs, profits, unforeseen expenses, etc.  

Handymen and management can give an estimate of expenses, but when turning over a unit, various problems that are unforeseen can come up.  

Also sounds like if you pay cash you will not have any reserves and I highly recommend having some cash reserves for the unexpected and unforeseen costs.